The apparatus of the present invention relates to automatic focus systems for optical imaging devices, particularly those which operate in the infrared and scan in a raster type pattern, for example, forward looking infrared systems ("FLIRs").
In thermal imaging systems, the scene may be defocused for a variety of reasons, for example, as a result of thermal and mechanical perturbations in the system, or as a result of actual variations in range to the scene. In high performance, low altitude airborne systems which are dependent on extensive sensor data, it is desirable to minimize pilot intervention in operation, thus reducing pilot workload. For a FLIR operating under conditions of variable range, thermal changes, and mechanical perturbations, this means that the sensor must contain an autofocus system.
Generally, mechanical perturbations are sufficiently minimized by design and construction techniques. However, thermal perturbations are more significant and difficult to remove, particularly in the case of a narrow field of view system using lens materials showing large changes in refractive index as a function of temperature. Focal instability caused by temperature variations might be minimized by use of different lens and mounting materials and mounting methods. However, this does not appear to be practical at this time.
A more practical method is to sense the temperature of the optics and to adjust the position of the focusing lens using a servo system. However, present temperature sensor systems are not accurate enough. Ultimately, the pilot has had to manually adjust focus, or systems have included a complex combination of temperature sensing and adjustment and image processing techniques.
One solution to the autofocus problem is described in the March, 1982 issue of the Scientific Honeyweller, Volume 3, No. 1, in the article entitled "Electronic Focus for Cameras", by N. Stauffer and D. Wilwerding, beginning at page 1. The described system, designed by Honeywell for 35 mm cameras (visible light), uses a separate set of detectors and charge coupled devices along with a microprocessor to effect automatic focus. A correlation is performed between sets of detectors utilizing two different, widely separated small areas of the aperture. The technique is based, as in optical rangefinders, on the angular difference between separate receivers in superimposing the same scene. This technique effectively determines not only whether the system is out of focus, but the amount and direction of lens movement required to achieve optimum focus.
Unfortunately this technique cannot be applied to a diffraction-limited system, such as a FLIR, because of the loss of image quality which results from using a small part of the aperture for focus information. In the case of a FLIR, the full aperture used for imagery must also be used to determine best focus. Consequently, an effective autofocus system for the FLIR would be based on processing the full video signal created by the scanning system and signal processing electronics. Best focus is characterized generally by the greatest image sharpness, which is equivalent to the maximum transfer of the high spatial frequencies in the video signal, or by the highest contrast at small objects or edges. Such contrast is represented by the slopes of the irradiance function in the image or video signal.
To use the video ouptut of the FLIR to determine optimum focus, it is necessary to compare the video signal when the focusing lens is located at different focal positions. One technique is to dither the focusing lens longitudinally along the optical axis of the system, process the signal so as to determine the focal bias, and then to restore a zero bias by moving the focusing lens to the appropriate position.
There are two limiting aspects of this approach. One lies in the use of a mechanical dither to provide a range of focal positions for the determination of the optimum focus. A passive system would be more attractive because dither results in blurring of the scene displayed on the video and because of design considerations involved due to mechanical movement. Secondly, the technique depends on a dither range which is large enough for focus discrimination, while still being small enough to provide acceptable imagery for continuous operation. These requirements are satisfied as long as the processor can extract the desired information without dither perceptible to the viewer. This design constraint results from the common use of the full video within a common time period, both for viewing and for focus determination.
It is, accordingly, a primary object of the present invention to provide an automatic focus system for an infrared imaging device which compensates for focal shifts resulting from range variations, temperature or mechanical perturbations, but which does not require dithering the focusing lens of the system to find the position of best focus, and does not distort or interrupt the viewing of the screen.